The present invention relates to an image forming apparatus equipped with a plurality of photosensitive members, and a belt arranged opposite to these photosensitive members.
An image forming apparatus such as a color laser printer is generally known, which comprises a plurality of development devices each storing different color toner, a plurality of photosensitive drums each of which is supplied with toner from the corresponding development devices via a developing roller, and a belt arranged opposite to the plurality of photosensitive drums. In this image forming apparatus, a recording sheet is conveyed on the belt and passes through the plurality of photosensitive drums, during which different color toner retained on the surfaces of the respective photosensitive drums is continuously transferred onto the recording sheet so that color printing is performed on the recording sheet.
To perform high quality image forming in this kind of image forming apparatus, Japanese Laid-open Patent Publication No. 2000-29365 discloses an image forming apparatus which comprises drum cleaners each slidingly contacting with a corresponding photosensitive drum to collect excess toner adhered on the photosensitive drum, a belt cleaner slidingly contacting with a belt to collect excess toner adhered on the belt, and a controller configured to reverse the polarity of a voltage applied to each drum cleaner at a predetermined timing. According to this image forming apparatus, the controller causes the voltage applied to the drum cleaner to be reversed between plus and minus at the predetermined timing, so that toner retained on the drum cleaner moves onto the photosensitive drum and is then transferred from the photosensitive drum onto the belt. Toner on the belt is finally collected by the belt cleaner.
In this conventional image forming apparatus, however, when toner retained on the drum cleaner is collected by the belt cleaner via the photosensitive drum and the belt with the use of potential difference between the electric charge of toner and the voltage applied to the drum cleaner, toner transferred from the upstream-most photosensitive drum onto the belt passes between the belt and three other photosensitive drums arranged downstream from the upstream-most one. Therefore, during the passage of toner; for example, when toner transferred from the upstream-most photosensitive drum onto the belt passes through a second and a third photosensitive drum, if the electric charge of toner becomes higher due to an electric discharge phenomenon that occurs by the potential difference for moving toner, toner is repulsed to each other and when the toner passes through downstream-side photosensitive drums, the toner moves onto these photosensitive drums such as a fourth photosensitive drum in particular.
In this instance, if the drum cleaner for the fourth photosensitive drum retains a large amount of toner, different color toner that has moved onto the fourth photosensitive drum is moved by the rotation of the photosensitive drum without being collected by the drum cleaner, and then the toner moves onto a developing roller and further to a development device, so that different color toner is mixed in the development device.
This is particularly serious if the development device is of a type where a developer receptacle forming a developer storage chamber and a developing unit forming a development chamber are separable, because the developer receptacle is replaced with a new one for filling up toner and different color toner is gradually increased in the development chamber of the non-replaceable developing unit. As a content of mixed toner, namely a ratio of different color toner to the mixed toner, increases, a quality of a produced image deteriorates, which leads to reduced service life of the development device in the end.
In view of the foregoing drawbacks of the prior art, the present invention seeks to provide an image forming apparatus of a type where a developer receptacle is attached to and detachable from a developing unit, and which can restrict reduced service life of the developing unit.